The present invention relates to an optical compensation film useful for application to a liquid crystal display, particularly to a twisted nematic liquid crystal display. It comprises two layers having different tilt angles and different retardation values within prescribed limitations.
The following terms have the definitions as stated below.
Optic axis herein refers to the direction in which propagating light does not see birefringence.
A-plate, C-plate and O-plate herein are the plates in which the optic axis is in the plane of the plate, perpendicular to the plate and tilted with respect to the plane of the plate, respectively.
Polarizer and Analyzer herein refer to elements that polarize electromagnetic wave. One closer to the source of the light is generally called polarizer while the one closer to the viewer is called analyzer.
Viewing direction herein is defined as a set of polar viewing angle xcex1 and azimuthal viewing angle xcex2 as shown in FIG. 1 with respect to a liquid crystal display 101. The polar viewing angle xcex1 is measured from display normal direction 103 and the azimuthal viewing angle xcex2 spans between an appropriate reference direction 105 in the plane of the display surface 107 and the projection 108 of the arrow 109 onto the display surface 107. Various display image properties, such as contrast ratio, color and brightness are functions of angles xcex1 and xcex2.
Azimuthal angle xcfx86 and tilt angle xcex8 are herein used to specify the direction of an optic axis. For the transmission axes of the polarizer and the analyzer, only the azimuthal angle xcfx86 is used, as their tilt angle xcex8 is zero. FIG. 2 shows the definition of the azimuthal angle xcfx86 and tilt angle xcex8 to specify the direction of the optic axis 201 with respect to the x-y-z coordinate system 203. The x-y plane is parallel to the display surface 107, and the z-axis is parallel to the display normal direction 103. The azimuthal angle xcfx86 is the angle between the x-axis and the projection of the optic axis 201 onto the x-y plane. The tilt angle xcex8 is the angle between the optic axis 201 and the x-y plane.
ON (OFF) state herein refers to the state with (without) an applied electric field to the liquid crystal display 101.
Isocontrast plot herein shows a change in a contrast ratio from different viewing directions. Isocontrast line, on which the contrast ratio is constant (such as 10, 50 and 100), is plotted in polar format. The concentric circle corresponds to polar viewing angle xcex1=20xc2x0, 40xc2x0,60xc2x0 and 80xc2x0 (outer most circle) and the radial lines indicates azimuthal viewing angle xcex2=0xc2x0, 45xc2x0, 90xc2x0, 135xc2x0, 180xc2x0, 225xc2x0, 270xc2x0 and 315xc2x0. The area enclosed within the isocontrast line with contrast ratio, for example, 10 is the viewing angle range with contrast ratio 10 or higher.
Lamination herein means a process of making a single sheet of film by uniting two or more films.
Liquid crystals are widely used for electronic displays. In these display systems, a liquid crystal cell is typically situated between a pair of polarizer and analyzers. An incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the analyzer. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. The energy required to achieve this control is generally much less than required for the luminescent materials used in other display types such as cathode ray tubes (CRT). Accordingly, liquid crystal technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, electronic games for which light-weight, low-power consumption and long-operating life are important features.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display (LCD) is the propensity for light to xe2x80x9cleakxe2x80x9d through liquid crystal elements or cells, which are in the dark or xe2x80x9cblackxe2x80x9d pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the direction from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle range centered about the normal incidence (xcex1=0xc2x0) to the display and falls off rapidly as the polar viewing angle xcex1 is increased. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction.
LCDs are quickly replacing CRTs as monitors for desktop computers and other office or house hold appliances. It is also expected that the number of LCD television monitors with a larger screen size will sharply increase in the near future. However, unless problems of viewing angle dependence such as coloration, degradation in contrast, and an inversion of brightness are solved, LCD""s application as a replacement of the traditional CRT will be limited.
Among various LCD modes, Twisted Nematic (TN) LCD is one of the most prevalent ones. FIG. 3A is a schematic of a TN-LCD 313. A liquid crystal cell 301 is positioned between a polarizer 303 and an analyzer 305. Their transmission axes 307, 309 are crossed, meaning that the transmission (or equivalently, absorption) axes of a polarizer and an analyzer form angle 90xc2x110xc2x0. Inside the liquid crystal cell, the optic axis of liquid crystal shows azimuthal rotation of 90xc2x0 in the OFF state across the cell thickness direction. In FIG. 3A, the direction of the liquid crystal optic axis 311 at the cell surfaces is indicated by a single head arrow. At the surface, the liquid crystal optic axes 311 have a small tilt angle xcex8s with respect to the cell surfaces in order to prevent the reverse twist. Namely, the tilt is consistent with the sense (clock or counter-clock wise) of the azimuthal rotation in the liquid crystal optic axis in the cell thickness direction. Un-polarized incoming light is linearly polarized by the polarizer 303 and its plane of polarization rotates 90xc2x0 while traveling through the liquid crystal cell 301. The plane of polarization of out-coming light from the cell 301 is parallel to the transmission axis 309 of analyzer 305 and will transmit through the analyzer 305. With sufficiently high-applied voltage, the liquid crystal becomes perpendicular to the cell plane except in the close vicinity of the bounding plates. In this ON state, the incoming polarized light essentially does not see birefringence and thus it is blocked by the analyzer. This mode (bright OFF state and dark ON state) is called Normally White Twisted Nematic Liquid Crystal Display (NW-TN-LCD) mode.
In the display normal viewing direction (xcex1=0xc2x0), one can obtain high contrast between the ON and OFF states. However, when the display is viewed from an oblique direction, the propagating light sees birefringence in ON state, thus it is not completely blocked by the analyzer. The isocontrast plot of the display 313 is shown in FIG. 3B. The lines 315, 317, 319 are isocontrast lines for contrast ratios 10, 50 and 100, respectively. The azimuthal angle xcex2 is measured from a reference direction 105 (shown in FIG. 1), which is chosen to be at 45xc2x0 relative to the transmission axis 307 of the polarizer 303 (namely, xcex2=45xc2x0, 225xc2x0 line corresponds to 307). The display fails to maintain contrast ratio 10 or higher in the range required for many applications. Viewing angle range with contrast 10 or higher is limited as the area enclosed within the isocontrast line 10, 315, is small. The isocontrast line 10 runs through xcex1=500 for the horizontal viewing direction (xcex2=0xc2x0, 180xc2x0). In the vertical direction, xcex1=30xc2x0, 58xc2x0 for xcex2=90xc2x0, 270xc2x0, respectively. Within the viewing angle range outside of the line 315, the contrast ratio between the On and the Off state becomes less than 10, or worse, a gray scale inversion occurs.
One of the common methods to improve the viewing angle characteristic of TN-LCD is to use the compensation films. In some cases, the compensation films consist of optically anisotropic layer deposited on the substrate. The substrate can be flexible film such as triacetyl-cellulose or rigid such as glass. The optically anisotropic layer is generally made of liquid crystal polymers. As it is necessary to align the optic axis of the liquid crystal polymer in the desired direction (making the anisotropic layer as A, C or O-plate), an alignment layer is often deposited between the optically anisotropic layer and substrate or between the two optically anisotropic layers. The thickness of the anisotropic layer depends on the property of its constituent material and LCD to which it is applied. The compensation films are typically inserted anywhere between the liquid crystal cell and the polarizers. The function of compensation film, in general, is to undo the retardation experienced by the propagating light while traveling through the liquid crystal cell. By using the compensation film in the ON state of NW-TN-LCD, birefringence experienced by the obliquely propagating light is cancelled by the film. This gives us uniform dark state resulting in improved viewing angle characteristic.
Various compensation methods have been suggested. U.S. Pat. No. 5,619,352 discloses the usage of combinations of O-plates. The basic idea is to compensate the ON state of TN-LCD by a stack of O-plates that have similar or complementary optical symmetries of the ON state liquid crystal cell. This was done by approximating the ON state of TN-LCD by three representative parts: two regions closed to the cell bounding plates and cell middle section. In the middle of the cell, the liquid crystal optic axis is almost perpendicular to the cell plane with large change in the azimuthal angle of liquid crystal optic axis. In the vicinity of the bounding plates, tilt in liquid crystal optic axis is small with almost fixed azimuthal angle. In comparison to the previous compensation technology, the stability of gray scale as well as viewing angle characteristic has been improved.
European Patent Application 1143271A2 discloses compensation film comprising two layers of O-plates. The tilt angle of optic axis increases continuously or stepwise in the film thickness direction in the first O-plate whereas it decreases continuously or stepwise in the second O-plate. These two O-plates are disposed on the substrate with negative C-plate property or biaxial plate having optical property close to that of negative C-plate. In negative C-plates, the ordinary refraction index is larger than the extraordinary one, and the optic axis is perpendicular to the plates. The film is used one side of the liquid crystal cell. The average tilt angle of two O-plates may assume different values. Examples 8 and 16, for example use combinations of average tilt angles 42xc2x0 and 31xc2x0, and 20xc2x0 and 89xc2x0, respectively.
Van de Witte et al. also disclosed in WO 97/44409 that the combination of two O-plates are used to compensate the Twisted Nematic Displays. The compensation films are applied for both sides of the liquid crystal cell. This case, too, the two O-plates can take different tilt angles, for example, 40xc2x0 and 50xc2x0. The tilt angles are preferably larger than 10xc2x0 but no more than 70xc2x0.
Chen et al. suggested (SID99, pp.98-101 xe2x80x9cWide-viewing-angle photo-aligned plastic films for TN-LCDsxe2x80x9d) the use of crossed O-plates to compensate the dark state of NW-TN-LCD. FIG. 4A shows structures of NW-TN-LCD 441 with O-plates 403 (also referred as optically anisotropic layer). In addition to the liquid crystal cell 401, four O-plates are placed between crossed polarizer 411 and analyzer 413. Optic axes 415 in all of four O-plates 403 tilt with the same angle, 20xc2x0 with respect to the plate plane. Also the retardations (or equivalently thickness, if all of the O-plates 403 are made of the same material) of the four O-plates 403 are the same and are 60 nm. The negative C-plates 421 represent the optical property of the triacetyl cellulose substrate. The retardation of the substrate Rsub with negative C-plates property defined by Rsub=(nesxe2x88x92nos)ds are set at xe2x88x9260 nm, where nes and nos are extraordinary and ordinary refraction indices of the substrate, respectively, and ds is the thickness of the substrate 421. The azimuthal angles of optic axes and transmission axes of analyzer and polarizer are given in the parenthesis. The azimuthal angle of the O-plate is equal to that of the transmission axes 423, 425 of the polarizer 411 or the analyzer 413. FIG. 4B shows the isocontrast plot of the display 441. The lines 427, 429 and 431 are isocontrast lines of contrast ratios 10, 50 and 100 respectively. With the O-plate compensation, the viewing angle characteristic was improved. The viewing angle range with contrast ratio 10 or higher (or, the area enclosed by the isocontrast line 10, 427 in the isocontrast plot shown in FIG. 4B) expanded compared to its counter part in the isocontrast plot shown in FIG. 3B.
The compensation film disclosed in the co-pending U.S. Pat. application Ser. No. 10/281,595 uses lower tilt angle and higher retardation values for O-plates compared to the display 441 (e.g., 10xc2x0 tilt angle and 100 nm for retardation). This replacement improved the vertical viewing characteristic of the display 441.
The prior art compensators using O-plate improved the viewing angle characteristic by reducing the light leakage in the ON state. However, viewing angle characteristic still remain unsatisfactory. For example, in the prior art display 441 shown in FIG. 4A, large viewing angle range with contrast ratio 10 or less (namely, the area outside of the isocontrast line 10, 427 in the isocontrast plot shown in FIG. 4B of prior art display shown in FIG. 4A) remains. The display with this viewing angle characteristic is certainly not applicable for LCD-TV or application that needs large viewing angle range, such as avionics displays. Also, prior arts do not focus on the systematic optimizations of parameters, e.g., the retardation and tilt angle combination along with the optical properties of the substrate. Thus, the range of parameters that offers functional compensation films is limited. Therefore there is a strong need for new compensation films that offer significantly higher contrast for wider viewing angle range than the displays with prior art compensation films. Also, the need is there to have a wider range of parameters for compensation films that lead to flexible manufacturing conditions.
The invention provides an optical compensation film for liquid crystal displays comprising, a first and a second optically anisotropic layers containing positively birefringent material disposed on a substrate, wherein the optic axis of said first optically anisotropic layer tilts in a first plane with an average tilt angle between 25xc2x0 and 70xc2x0, and the optic axis of said second optically anisotropic layer tilts in a second plane with an average tilt angle between 0xc2x0 and 40xc2x0, and said average tilt angle of said first optically anisotropic layer and said average tilt angle of said second optically anisotropic layer are different, and said first and said second planes are perpendicular to the plane of said optical compensation film with the angle between said first and said second planes being 90xc2x110xc2x0, and the retardation defined by (ne1xe2x88x92no1)d1of said first optically anisotropic layer is between 50 nm and 160 nm and the retardation defined by (ne2xe2x88x92no2)d2 of the said second optically anisotropic layers is between 0 nm and 80 nm, and said retardation of said first optically anisotropic layer and said retardation of said second optically anisotropic layer are different, where ne1 and no1 are extraordinary and ordinary indices of refraction of said positively birefringent material of said first optically anisotropic layer, respectively, and ne2 and no2 are extraordinary and ordinary indices of refraction of said positively birefringent material of said second optically anisotropic layer, respectively, and d1 and d2 are thickness of the said first and said second optically anisotropic layer, respectively.
The compensation film can be used in conjunction with a twisted nematic liquid crystal display for an improved viewing angle characteristic. The invention also includes a display incorporating the film of the invention.